This invention relates to metal shear panel fasteners, specifically nails and screws used to fasten shear resistant diaphragms to framing elements in the construction of framed buildings and the like and to joints and structures created therewith.
All buildings require shear resistant structural elements in order to resist lateral forces produced by winds and earthquakes. In general, there are three types of structural systems used in framed buildings to resist lateral shear forces: moment resistant frames, diagonal bracing, and shear resistant diaphragms. A typical shear wall assembly consists of three structural elements: a frame bounds the shear panel and provides intermediate elements to inhibit panel buckling and increase tensile and compressive strength; shear panels are applied over the frame to provide shear resistance; and a fastening system which connects the shear membrane to the frame elements.
Typically shear panels are constructed from sheets of plywood or Oriented Strand Board (OSB), thin sheet steel, or other composite materials (such as reinforced plastics) applied to wood or steel studs or joists by means of nails or screws. These panels are then usually covered with sheets of Gypsum Wall Board (GWB) or some other finish materials to provide a cosmetically attractive surface. When the anticipated loads are low enough the Gypsum Wall Board has sometimes been used by itself to provide the structural membrane.
Until recently, the laboratory structural testing used to certify the strength of shear resisting systems has employed a gradually applied, one directional, load (so called monotonic loading). However, as more experience with failures in actual earthquake conditions has been acquired, monotonic certification has come into question. Brittle materials have been determined to perform poorly under cyclical, shock loading conditions. Pending the development of more realistic testing methodology the allowable strengths of GWB has been halved in the Uniform Building Code.
The use of GWB, plywood, or OSB for shear walls pose structural problems for design engineers and installers:
(a) Plywood and OSB have a lack of uniformity in strength due to variations in wood properties and manufacturing processes. Variations i n construction conditions such as overdriven nails and exposure to moisture also reduce the strength of assembled shear walls. PA1 (b) GWB panels are very brittle and the paper covering has very little tearthrough strength. Under cycled loading conditions the screws or nails pull though the face paper and the gypsum surrounding the fasteners fractures at relatively low loads. PA1 (c) The hardened steel screws sometimes used as fasteners also perform poorly compared to nails when used with high strength membranes such as plywood. The hardening process reduces ductility and the threads introduce stress concentrations. Under cyclical loading conditions the screws fatigue and fail near the surface of the framing elements. PA1 a! to provide a fastener which bears directly on the structural membrane to increase tear-through strength PA1 b! to provide a fastener bearing face close to the framing element so as to reduce fastener bending stresses PA1 c! to provide a fastener bearing face close to the framing element so as to reduce fastener withdrawal loads PA1 d! to provide ductile fastener PA1 e! To provide a fastener that can be rapidly installed using a pneumatically actuated nail gun or electrically operated screw gun PA1 f! to provide an increased bearing area in the structural membrane to increase its lateral load capacity and energy capacity PA1 e! to provide increased joint ductility and energy absorption so as to reduce building loads by hysteretic damping PA1 f! to provide a self gauging means of fastener installation and inspection PA1 g! to allow the use of a smaller diameter lower portion of the shaft for the fastener so as to reduce splitting in wooden framing elements when large numbers of fasteners are required for high load conditions
In 1977, U.S. Gypsum was granted U.S. Pat. No. 4,016,697 for gypsum wall board clad with sheet steel on one face for use in buildings. That patent shows the attachment of the composite panel to the framing elements by means of conventional screws, nails, and clips. The patent discusses the use of the panels as bending or compression elements but makes no mention of shear capability. Recently a structural panel identical to U. S. Gypsum's patented system has been marketed by Cemco for use as a shear panel in steel framed buildings. Cemco's panel consists of a sheet of GWB bonded to a sheet of either 22 gauge or 25 gauge sheet metal. This panel is designed to be screwed to a steel frame and eliminates the need for a separate lateral bracing system. Panels of the foregoing type suffer from structural limitations under conditions of earthquake loading due their employment of conventional fasteners. Under cycled loading conditions the screws' heads tend to pull through the paper facing of the GWB under the influence of out-of-plane loads. When the structural membrane begins to lift free of the face of the studs the resulting increased bending load on the screws, combined with the screw's fatigue problem, result in a premature structural failure. Nails could be substituted for the screws but the tear-through problem remains. Screws could be driven completely through the gypsum wall board so as to bear directly on the steel but the fatigue problem remains and inspection is difficult. Both nails and screws provide limited bearing area to resist tearout/bearing failure in thin structural membranes. At present there is no fastener available that fully utilizes the structural capabilities of thin, shear membranes. Conventional screws and nails as well as specialty fasteners were not designed for, nor are they suitable, for this application:
So called "duplex" nails are used in concrete form work to facilitate the subsequent reuse of wooden forming materials. Such fasteners, described in U.S. Pat. No. 451,213 to Shepley, are double headed nails to be used for the temporary erection of wood. These fasteners are not intended to be driven deeper than their lowermost head so that the upper head is readily accessible for later extraction of the form nail. Duplex nails provide no greater bearing area in shear than standard nails, are not available in appropriate head configurations for this application, and lack the proper relationship between the head's locations so as to be self gauging during installation.
So called "ring-shanked" nails are used to provided increased resistance to nail withdrawal in wood flooring systems. Such fasteners are described in U.S. Pat. No. 2,172,553 to Tripp. The annular ridges of such fasteners are of a constant diameter and would produce an unsuitably oversized hole in the structural membrane and weakening the shear and uplift strength of the connection.
A specialty nail for the installation of plaster rock of lathing disclosed in U.S. Pat. No. 2,633,049 to Anderson has a moveable head and a conical projection on the shank intended to be driven completely through the rock layer and into the wood stud to create an oversized hole in the rock and allow subsequent movement of the rock lath. Such a fastener would produce serious structural damage to a structural membrane and would create a weak attachment.
A specialty nail for attaching metal lathing to wood studs disclosed in U.S. Pat. No. 383,951 to Hegbom in 1888 has a protrusion of the nail shaft for the purpose of supporting the lath. The nail's shoulder is located well clear of the surface of the stud and would provide no structural advantage over a standard nail in this application.
If a water resistant, fire resistant, GWB panel with a structural membrane backing were provided with an easily installed and inspected fastening system which took full advantage of the strength and ductility characteristics of the structural membrane, the supporting frame, and the fastener then a marked advance in the art of framed building construction would ensue.
The improved shear resistant panel assembly forming the subject of this invention is best accomplished by a novel panel of water and fire resistant GWB bonded to a layer of galvanized sheet metal said panel attached to the framing elements by shouldered fasteners which pin the sheet metal directly to the face of the frame. The structural backing can also be thickened along the panel edges to increase its resistance to fastener tearout under high loads. The structural backing can also be constructed of other materials such as fiber reinforced resins. The GWB layer can also be replaced by other finish materials such as wood or plastic.